Compensating the effects of static head-media spacing variations and nonlinear transition shift in heat assisted magnetic recording

ABSTRACT

An apparatus comprises a storage medium, a recording head, a source of electromagnetic radiation, and a control circuit for modulating the source of electromagnetic radiation in response to a static deviation of a spacing between the recording head and the storage medium. A method of compensating a static deviation of a spacing between a recording head and a storage medium performed by the apparatus, and a method of precompensating for nonlinear transition shifts in a heat assisted magnetic recording system, are also provided.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underAgreement No. 70NANB1H3056 awarded by the National Institute ofStandards and Technology (NIST). The United States Government hascertain rights in the invention.

FIELD OF THE INVENTION

This invention relates to data storage devices and more particularly tomethods and apparatus that compensate for the effects of variation instatic head-media spacing in data storage devices.

BACKGROUND OF THE INVENTION

A typical disc drive includes a housing that encloses a variety of discdrive components. The components include one or more rotating discshaving data surfaces that are coated with a medium for storage ofdigital information in a plurality of circular, concentric data tracks.The discs are mounted on a spindle motor that causes the discs to spin.Each rotating disc has a corresponding head gimbal assembly (HGA). TheHGA includes a slider, which carries a transducer that writesinformation to and reads information from the data surfaces of thediscs. The slider and transducer are often together referred to as the“head.” The HGA also includes a gimbal that allows the slider to pitchand roll while following the topography of the disc. An actuatormechanism moves the HGAs from track to track across the surfaces of thediscs under control of electronic circuitry. The actuator mechanismincludes a track accessing arm and a suspension for each HGA. Thesuspension includes a load beam. The load beam provides a preload force,which forces the slider toward the disc surface.

During operation, as the discs rotate, the discs drag air under therespective sliders and along their bearing surfaces in a directionapproximately parallel to the tangential velocity of the discs. As theair passes beneath the bearing surfaces, air compression along the airflow path causes the air pressure between the discs and the bearingsurfaces to increase, creating a hydrodynamic lifting force thatcounteracts the load force provided by suspensions. The hydrodynamiclift force causes the sliders to lift and fly above or in closeproximity to the disc surfaces.

In a magnetic recording system it is desired to keep the magnetic headat a known constant distance from the magnetic medium surface in orderto meet overall system performance and reliability measures. For thispurpose, air bearing designs should take into account the given head andmedia specifications to compensate any deviations from the desiredheight. However, the magnetic head does not always fly over the mediumof interest with a desired Head-Media Spacing (HMS), but rather deviatesfrom this desired value. There are two main components of HMS deviationfrom the desired value.

Static HMS deviations result from manufacturing variations in head andmedia combinations. In general, each head will fly at a differentaverage height over the medium. The average fly height is also afunction of the radius at which the head is flying. For a given radius,the difference between the mean fly height of any head/media pair andthe desired HMS is defined as static HMS variation.

Dynamic HMS deviations cause the HMS to vary about the mean fly heightdue to factors such as compressibility of the air bearing, asperities onthe medium, excitation of the suspension, and gimbal modes on which thehead is mounted, etc. Dynamic HMS variation is defined as the variationin fly height about the mean fly height for a given head and mediumcombination at a given radius.

In one conventional recording system, the mean static HMS was measuredat 17.05 nm, with a standard deviation of 0.34 nm. This variation islarge enough to cause poor system performance and reliability in someproduction line samples. The HMS values for those samples can bedetected, and a compensation mechanism can be applied to those samplesto correct for deviations from the desired HMS. A known compensationmechanism is based on applying heat to the write head prior to writingin order to cause the pole tip of the writer to protrude from the headto achieve the desired static HMS. That technique requires heat that isproduced using the preamp in the data storage system to power a heateron the head. The amount of heat to be applied as a function of the discradius is determined during a factory calibration routine.

In conventional (longitudinal and perpendicular) magnetic recording,whenever the applied field is larger than the coercively (H_(c)) of themedium, the medium will be magnetized towards a +M_(r) (positiveremanent magnetization) direction (i.e., magnetized left or up), andsimilarly if the applied field is smaller than −H_(c) the magnetizationwill be towards a −M_(r) direction (i.e., magnetized right or down).

However, the conventional magnetic recording architectures are limitedby well-known super paramagnetic limits. Heat Assisted MagneticRecording (HAMR) uses a medium with very high coercively H_(c) to makesure that the medium is thermally stable with very small grain volumesV. The coercively is reduced during the write process by heating themedium, for example with a focused laser beam. Once the medium isheated, the reduced coercively makes writing possible. Then, afterwriting the bit, the medium cools back to its original temperature withhigh coercively H_(c) allowing the medium to be thermally stable.

There is a need for a HMS compensation method that can be applied toheat assisted magnetic recording.

SUMMARY OF THE INVENTION

This invention provides an apparatus comprising a storage medium, atransducer, a source of electromagnetic radiation, and a control circuitfor modulating the source of electromagnetic radiation in response to astatic deviation of a spacing between the transducer and the storagemedium.

In another aspect, the invention provides a method of compensating astatic deviation of a spacing between a transducer and a storage medium.The method comprises: producing a control signal representative of thestatic deviation in spacing between the transducer and the storagemedium, and modulating a source of electromagnetic radiation to heat aportion of the storage medium in response to the control signal.

The invention further encompasses a method of compensating a nonlineartransition shift in a heat assisted magnetic storage system. The methodcomprises: producing a control signal representative of nonlineartransition shift, modulating a source of electromagnetic radiation toheat a portion of the storage medium in response to the control signal,and applying a magnetic field to the storage medium to cause magnetictransitions in the storage medium, wherein transition locations in thestorage medium are changed by changing a temperature profile in thestorage medium.

The invention also encompasses a method of compensating for disturbancesin a storage device. The method comprises: comparing a read signal withan optimum read signal parameter to produce a control signal, andmodulating a source of electromagnetic radiation to heat a portion of astorage medium in response to the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of the mechanical portion of a discdrive that can be constructed in accordance with an embodiment of theinvention.

FIG. 2 is a schematic representation of a recording head that can beused in apparatus constructed in accordance with an embodiment of theinvention.

FIGS. 3, 4 and 5 are schematic diagrams that show various Head-MediaSpacing (HMS) parameters.

FIG. 6 is a functional block diagram of a prior art HAMR system.

FIG. 7 is a schematic representation of a sector format.

FIG. 8 is a functional block diagram of a HAMR system constructed inaccordance with an embodiment of the invention.

FIG. 9 is a schematic representation of radial variation in an AGCsignal.

FIG. 10 is a graph illustrating isolated transition responses fordifferent HMS values for a fixed temperature.

FIG. 11 is a graph illustrating isolated transition responses fordifferent head fields for a fixed temperature.

FIG. 12 is a graph illustrating isolated transition responses fordifferent temperatures with a fixed head field.

FIG. 13 is a graph illustrating isolated transition responses fordifferent head fields and temperatures.

FIG. 14 is a schematic representation of AGC variation for anon-uniformly coated media.

FIG. 15 is a graph of laser power versus drive current.

FIGS. 16 and 17 show an a-parameter profile along the cross-trackdirection together with a system transition response for two peaktemperatures.

FIG. 18 is an example of a graph of transition shift as a function oftemperature for different deep-gap field values in a write head.

FIG. 19 is another example of a graph of transition shift as a functionof temperature.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, FIG. 1 is a pictorial representation of themechanical portion of a disc drive 10 that can be constructed inaccordance with the invention. The disc drive includes a housing 12(with the upper portion removed and the lower portion visible in thisview) sized and configured to contain the various components of the discdrive. The disc drive includes a spindle motor 14 for rotating at leastone data storage medium 16 within the housing, in this case a magneticdisc. At least one arm 18 is contained within the housing 12, with eacharm 18 having a first end 20 with a transducer in the form of arecording and/or reading head or slider 22, and a second end 24pivotally mounted on a shaft by a bearing 26. An actuator motor, whichmay be a voice coil motor 28, is located at the arm's second end 24, forpivoting the arm 18 to position the head 22 to a desired position. Theactuator motor 28 is controlled by a controller that is not shown inthis view. The disc includes a plurality of servo sectors 30 arrangedbetween a plurality of data sectors 32. The data and servo informationis contained in a plurality of tracks 34.

FIG. 2 is a schematic representation of a heat assisted magneticrecording head 40 that includes an optical transducer, in combinationwith a magnetic recording medium 42, that can be used in an apparatusconstructed in accordance with this invention. Although FIG. 2 shows aperpendicular magnetic recording head and a perpendicular magneticrecording medium, it will be appreciated that the invention may also beused in conjunction with other types of recording heads and/or recordingmediums where it may be desirable to employ heat assisted recording. Therecording head 40 in this example includes a writer section comprising amain write pole 44 and a return or opposing pole 46 that aremagnetically coupled by a yoke or pedestal 48. A magnetization coil 50surrounds the yoke or pedestal 48 for energizing the recording head 40.The recording head 40 may also include a read head, not shown, which maybe any conventional type of read head as is generally known in the art.

Still referring to FIG. 2, the recording medium 42 is positionedadjacent to or under the recording head 40. The recording medium 42includes a substrate 52, which may be made of any suitable material suchas ceramic glass or amorphous glass. A soft magnetic underlayer 54 isdeposited on the substrate 52. The soft magnetic underlayer 54 may bemade of any suitable material such as, for example, alloys ormultilayers having Co, Fe, Ni, Pd, Pt or Ru. A hard magnetic recordinglayer 56 is deposited on the soft underlayer 54, with the perpendicularoriented magnetic domains contained in the hard layer 56. Suitable hardmagnetic materials for the hard magnetic recording layer 56 may includeat least one material selected from, for example, FePt or CoCrPt alloyshaving a relatively high anisotropy at ambient temperature.

The recording head 40 also includes a planar waveguide 58 that directslight received from a light source onto a surface of a recording mediumto heat the magnetic recording medium 42 proximate to where the writepole 44 applies the magnetic write field H to the recording medium 42.The planar waveguide includes a light transmitting layer 60. The opticalwaveguide 58 acts in association with a light source 62 which transmitslight, for example via an optical fiber 64, that is coupled to theoptical waveguide 58, by a coupling means such as a grating 66. Thelight source 62 may be, for example, a laser diode, or other suitablesource of electromagnetic (EM) radiation. This provides for thegeneration of a guided mode that propagates through the opticalwaveguide 58 toward the recording medium. EM radiation, generallydesignated by reference number 70, is transmitted from the waveguide 58for heating the recording medium 42, and particularly for heating alocalized area 72 of the recording layer 56.

FIGS. 3, 4 and 5 are schematic diagrams that show various Head-MediaSpacing (HMS) parameters. In FIG. 3, an arrow represents an air bearingsurface 80 of a recording head that is spaced at a desired head-mediaspacing 82 above a storage medium 84. FIG. 4 shows alternative averageair bearing surface positions as arrows 86 and 88. In the averageposition indicated by arrow 86, the static variation in HMS is shown asarrow 90. In the average position indicated by arrow 88, the staticvariation in HMS is shown as arrow 92. FIG. 5 shows a dynamic change inHMS as line 94. The range of the dynamic variation in HMS is shown asarrow 96.

In Heat Assisted Magnetic Recording, heat is applied through a lightdelivery system directly to the medium of interest during the writingprocess, and the applied heat changes the coercively profile of themedium that is directly under the head, thus changing the overall systembehavior.

A generic prior art HAMR system 100 is shown in FIG. 6. User data 102 isinput to an encoder 104 and subjected to a HAMR write process 106.During writing, a laser 108 is used to heat the recording medium totemporarily reduce the coercively of the medium. When the data is to beread, a read head produces a read signal in a read process 110. The readsignal is passed to a variable gain amplifier 112, filtered in a filter114 and converted to a digital signal in an analog-to-digital converter116. The digital signal is equalized in an equalizer 118 and passed to adetector 120 for a subsequent decision 122 to produce an estimate of theuser data 124. A timing recovery circuit 126 receives signals from theequalizer output and the detector to produce a signal on line 128 thatcontrols the analog-to-digital converter. The timing recovery circuitalso produces a signal on line 130 that is used by an adaptive gaincontroller 132 to control the variable gain amplifier 112.

In the system of FIG. 6, heat is present in the system while writingencoded user data to the medium (in the write process). During the readprocess, analog readback signals are produced by read head and sent to aread channel.

The encoded user data is processed as sector-by-sector data. A genericsector format 134 is shown in FIG. 7. During the read process, thereadback signal corresponding to the “PLL/AGC Field” 136 at thebeginning of each sector is used to adjust both the gain of the variablegain amplifier (VGA) through the adaptive gain controller (AGC) and alsothe sampling instants for the analog-to-digital converter (A/D) throughthe timing recovery (TR) block in FIG. 6. The outputs of the AGC andtiming recovery blocks estimate the average signal amplitude and thephase of the noisy readback signal obtained after the read process.Then, the readback signal corresponding to the “Sync Mark” field 138 isprocessed to make sure that the system can detect the known patternwritten at that field. Once the sync mark is detected, the read channelarchitecture is assumed to be ready to process the encoded user data140.

This invention detects the amount of static HMS in the system bymeasuring the magnitude of the signal read by the read channel. Then,the laser power is changed to alter the physical channel operating pointin order to compensate any static HMS variations from the ideal value.

The coercively profile of the medium in HAMR systems depends on thetemperature profile applied to the storage medium during writing. Thus,the coercively at any point of the medium will be a function of thetemperature value at that point, i.e., a higher temperature results in alower coercively.

A transition in the direction of magnetization of the media occurswhenever the applied field strength is equal to the coercively of themedium. However, the temperature profile also defines the location ofthe transition. In other words, the transition location in the storagemedium on a HAMR system can be changed by changing the temperatureprofile created by light supplied from the laser source.

A system constructed in accordance with the invention is shown in FIG.8. The system of FIG. 8 includes two additional blocks that are notfound in the generic system of FIG. 6. A first one 144 of the additionalblocks computes the average magnitude of the read signal over M sectors.This block uses the output of the adaptive gain controller (AGC) as itsinput. Since the fly height of the reader is correlated to the strengthof the readback signal, the output of the AGC can be used to inferstatic HMS for a given radius. In practice, the AGC output can be noisyand vary considerably from sector to sector due to non-uniformities inthe medium and dynamic HMS variations. The output of block 144 is asignal representative of the average HMS (the static HMS parameter).This signal is then used to modulate the power of the laser. Once theamount of static HMS has been estimated, the “Find Laser Power” block146 finds the amount of laser power for a given radius, whichcompensates the static HMS effects. In the “Find Laser Power” block,mapping from the static HMS parameter to the laser power amount can beimplemented using a look-up table.

FIG. 9 shows an example AGC variation pattern as a function of the headlocation on the disc surface. As can be seen from FIG. 9, for a givenradius, the AGC value may fluctuate. Thus, averaging the AGC output overM sectors will reduce these effects, and will also eliminate dynamic HMSvariations. The number M is a function of the amount of randomfluctuations in the system, and the averaging should not be done if afatal distortion (such as thermal asperity) or nonlinearity is detected.FIG. 9 also shows how the AGC values may change as a function of radius,thus averaging should also be done at different radial locations.

In order to illustrate the effectiveness of the invention, a simulatorhas been used to obtain signal magnitudes corresponding to a dibit atdifferent HMS values at a fixed temperature. The simulation resultsshown in FIG. 10 suggest that 2 nm of difference from a desired value of15 nm may result in a very big difference in the system dibit responseenergy, which directly translates into the signal energy andSignal-to-Noise Ratio (SNR) of the system. This means that the rest ofthe read channel architecture already designed for a desired HMS value(for example, 15 nm in this example) can either suffer from degradedperformance (because of lower SNR when the HMS increases) and/or fromrobustness (because of the change in channel response). A similarbehavior is found with the Thermal Williams-Comstock model asillustrated in FIG. 11, which shows the isolated transition responsesfor different head fields at a fixed temperature. The amounts of thehead fields in FIG. 11 are arranged to show a similar trend as shown inFIG. 10.

In a HAMR system, the amount of heat transferred to the media alsochanges the channel response. This is illustrated in FIG. 12. Thus,changes of the channel response in FIG. 12 can be compensated byadjusting the temperature profile in the system. This is illustrated inFIG. 13.

The simulation did not consider the track width increase, thermalprofile Full-Width-Half-Maximum (FWHM) increase, or thermal gradientdecrease as a function of HMS while obtaining the simulation resultsshown in FIG. 13. With these effects present, the static HMS changes inthe system may not be fully recoverable. Nevertheless, some or majorpart of the static HMS variations will be recoverable. The system can betuned once, for example during the power-up of the hard drive at thevery beginning of its operation, to set the laser power at each radius.Then, the same laser power can be kept for a given radius if the staticHMS does not change over time for a given hard disk architecture.

Several embodiments of the invention can be practically implemented,depending on the nature and magnitude of the HMS variation contributorssuch as operating temperature, pressure, mechanical expansion ofmaterials, change in laser power over time or due to alignment shifts,and pole tip wear on the head. For example, if the operating radius andtemperature are the most significant contributors to HMS variation, amanufacturing process can calibrate and save the required laser powertables to non-volatile drive memory (or even somewhere in a reservedarea on the drive that can be read upon power-up). If some of theaforementioned variation contributors can change while a drive is inservice, then periodic self-calibration operations may be executed bythe drive to keep the HMS constant. Such methods are routinely employedin conventional drives to keep critical drive parameters in calibrationover each drive's service life.

In addition, changes in the HMS may also occur due to coatingvariations. FIG. 14 shows an example of a non-uniformly coated media.The AGC is once again an indicator of the readback amplitude, which ismodulated by the media coating variation. This information can becompressed in both radial and circumferential dimensions and stored in alook-up table. Then the information can be applied using the methoddescribed above to compensate for systematic radial and circumferentialchanges in coating uniformity.

This invention addresses the effects of variations in static HMS byproperly choosing the laser power in a HAMR system. A system thatimplements the method is simple to implement and effective ineliminating the effects of static HMS variations. The system can betuned at first at different radial locations to account for any slowlychanging HMS variations (for example due to non-uniformities in mediacoatings). Then, it can be periodically tuned for time varyingparameters, such as temperature changes, laser aging, couplingefficiency changes, changes in media uniformity, and such, to increasethe overall system performance.

Modulating the temperature profile not only changes the location of themagnetic transitions in the storage medium, but also alters thetransition width of the system. This may cause signal energy loss, anincrease in Inter-Symbol-Interference (ISI), or miss-equalization atoff-track values. Thus, it is important to choose an operatingtemperature range that will produce the desired results withoutintroducing any other unwanted effects.

One way to chose an appropriate operating temperature range is to findthe nominal peak laser power (i.e., the nominal peak temperature in thesystem) which gives less reduction in signal energy, reasonable ISI, andsmall miss-equalization effects at practical off-track range ofinterest, and does not degrade the head disk interface. Then, find therange of the peak laser power around its nominal value (i.e., the rangeof the peak temperature in the system which is used to adjust thetransition shifts) so that the change in signal energy, ISI, andmiss-equalization at off-track positions within that temperature rangeare minimum.

During the write process of magnetic recording, location of the writtenbits can be shifted due to demagnetization fields of the adjacent bits.This data dependent nonlinear shift is called Nonlinear Transition Shift(NLTS), and creates unwanted distortion in the system. NLTS degrades themagnetic recording system performance, and write precompensation blocksare available in conventional read channel chips to cancel this effect.

As the Heat Assisted Magnetic Recording (HAMR) system is essentially amagnetic recording system, this unwanted transition shift effect mayalso be present in HAMR systems. Moreover, since the envisioned arealdensity for HAMR is much higher than the areal densities of today'scommercial products, the effect of NLTS may be even worse.

A first look-up table of information representative of the relationshipbetween the data pattern and the amount of shift caused in the systemcan be established. Similarly, a second look-up table of informationrepresentative of the relationship between the amount of shift to becompensated and the laser power profile corresponding to that particularshift can also be established.

The process can then proceed as follows. At time instant k, get the bitsa_(k−L),ak_(−L+1) . . . a_(k) . . . a_(k+L−1),a_(k−L), where it isassumed that only the bits within a span of ±L samples affect thetransition at time k.

Using the first look-up table, identify the amount of shift caused bybits a_(k−L),a_(k−L+1) . . . a_(k) . . . a_(k+L−1), a_(k+L) on thetransition of interest at time k.

Using the second look-up table, map the amount of shift in the laserpower profile that compensates the particular transition of interest,and also compensates for any excessive ISI in the system (if there isany). Then write the transition at time k, with the specified laserpower profile.

The method can be simplified by combining the two look-up tables intoone look-up table. However, the method has been described using twolook-up tables for the sake of clarity in explanation.

Successful performance of the laser modulation method described abovedepends on how well the laser can be modulated. There are a number ofdifferent techniques for actively changing the laser power during therecording process. Some are better suited to an integrated HAMR drive,while others are better suited to a spin stand environment. For the sakeof completeness, multiple modulation methods are described below.

Modulating the laser by modulating the drive current directly is mostlikely the best solution for an integrated HAMR drive. The amount oflaser power emitted from a laser diode is proportional to the drivecurrent as shown in FIG. 15. Increasing or decreasing the drive currentcan easily control the output power. Many laser driver chips exist todaythat are specifically designed to modulate laser diodes and boast riseand fall times under 1 ns. The chips also allow for different laserpulsing schemes, which are data dependent. For example, in many CD andDVD recording schemes the laser driver pulses the laser in order tocontrol the mark shape and prevent bloom.

FIG. 15 is a graph of laser power versus drive current. In FIG. 15,region A is the LED emitting region, and region B is the laseroscillating region. For this scheme to be implemented in a HAMR drive,an integrated circuit (IC) chip could be designed with a built-inlook-up table so the correct laser modulation scheme could be matched upwith the data pattern that is to be recorded.

In an acoustic optical modulation system, an acoustic wave is used toscatter the laser beam into higher order diffraction modes. By changingthe amplitude of the acoustic pulse, more or less light can be scatteredinto a particular mode. Optical power is modulated by changing the powerscattered into a particular mode. Acoustic modulators are limitedprimarily by the amount of power they can handle (less than 100 W/mm²)and the rise and fall times of the modulation (˜10 ns). To integrate anacoustic optical modulator into a HAMR drive, a fused silica crystalcould be placed in the path of the laser. In addition, special driveelectronics would be required in order to control the crystal.

Electro-optic and magneto-optic modulation methods are very similar.Although there are many different schemes to realize modulation usingthese techniques, they operate on similar principles. In both cases, anapplication of a magnetic field or electric field to a specific crystalinduces a change in the refractive index of the material, which causesthe polarization of the light to be changed. By using an analyzer inconjunction with the crystal, the amount of light that passes throughthe modulator can be controlled. Both types of modulators are generallyfaster than acoustic optical modulators and can handle slightly morepower. However, both are still slower than direct current modulation ofthe laser diode itself.

Another means for controlling the amplitude of the laser light is aliquid crystal.

Laser modulation can also be achieved by deflecting the optical beam.The coupling efficiency of light into the waveguide using the couplinggrating is strongly dependent on the position of the incident spot onthe grating. By adjusting the beam position, the coupling efficiency canbe either decreased or increased. One method for steering the beam wouldbe to actively tilt the laser diode or by bouncing the beam off of amirror and changing the angle of a mirror. It is also possible todeflect a beam using the electro-optic, magneto-optic and acousticoptical effects.

Compensation has been simulated assuming ideal laser power modulation.Two independent HAMR signal modeling methods were used. The first oneimplements the Thermal Williams-Comstock Model together with microtrackmodeling, and the second one involves a micromagnetic modelingtechnique.

A Thermal Williams-Comstock Model was used together with microtrackmodeling to analyze the effect of temperature profile onto systembehavior. During simulations, the temperature profile was assumed to beGaussian distributed, and the same temperature profile was applied bothalong down-track and cross-track directions.

The coercively H_(c) and remanence magnetization M_(r) was assumed to belinearly dependent on the temperature. The head field was determinedusing the following equation:

$H_{x} = {\frac{H_{0}}{\pi}\left\lbrack {{\tan^{- 1}\left( \frac{x + \frac{g}{2}}{y} \right)} - {\tan^{- 1}\left( \frac{x - \frac{g}{2}}{y} \right)}} \right\rbrack}$where g represents the width of the gap, and y stands for the distancefrom the medium to the head. During the simulations, g was assumed to be100 nm, and y to be 20 nm. During read operation, g was changed to be 5nm in order to be within the assumptions of the ThermalWilliams-Comstock Model. Although it might not be realizable to haveread head designs which have effective 5 nm gap width, the results showa trend observed in a typical HAMR system.

First assume the coercively and remanent magnetization dependencies onthe temperature asH _(c)=−2000T+1.6×10⁶M _(r)=−1500T+1.2×10⁶.

FIGS. 16 and 17 are graphs of the changes in the a-parameter andtransition response with temperature (H₀ of the head field is 2.4Tesla), where the a-parameter provides an indication of transitionwidth. FIGS. 16 and 17 show the a-parameter profile along thecross-track direction together with the system transition response fortwo peak temperatures. FIGS. 16 and 17 assumed the head field with H₀equal to 2.4 Tesla, the peak temperature was taken as 700° K. (Kelvin)and 600° K. (Kelvin), Bit-Aspect-Ratio (BAR) was assumed to be 5, andNormalized Density (ND) was set to 2. The a-parameter profile and systemtransition response does not change significantly with temperaturechanges. This means that the change in signal energy, ISI, and channelmiss-equalization at off-track values do not change much as thetemperature changes from 600° K. to 700° K. Thus for this specific case,a temperature range can be found that satisfies the first item in thealgorithm.

Next, the change in transition location at the center of the track as afunction of temperature was examined. Simulations have been performedassuming that the peak temperature is not modulated within the durationof bit length. FIG. 18 is a graph of transition shift as a function oftemperature for different deep-gap field values in the write head. FromFIGS. 16 and 17 a temperature range between 600° K. and 700° K. has beenidentified where there is not much change in signal energy, ISI, andmiss-equalization at off-tracks. Assuming that the temperature value iscentered at 650° K., the location of the transitions can be changed by 5nm (for the three deep-gap field values considered) within a ±50° K.temperature range. Thus, for a bit width equal to 11 nm (whichcorresponds to a 1 Tbpsi design for a BAR equal to 5), this correspondsto precompensating the transition shifts around 45% by only modulatingthe peak temperature within ±50° K. range. Of course, more compensationcan be provided with a larger range of temperatures.

If the coercively and remanent magnetization dependencies on thetemperature are assumed to be:H _(c)=−500T+80×20×10³M _(r)=−300T+80×15×10³,where the coercively and remanent magnetization are less sensitive totemperature changes, the plots in FIG. 19 are obtained. Again, FIG. 19is a graph of transition shift as a function of temperature, assuming acenter temperature value of 650° K. In this case, the location of thetransitions change by 2 nm (for a deep-gap field equal to 2.4 Tesla) to6 nm (for a deep-gap field equal to 2 Tesla), within a ±50° K.temperature range, which correspond to around 18% to 55% for bit widthequal to 11 nm.

The precompensation block specifications in conventional read channelarchitectures indicate that it can compensate up to 10/64 (approximately16%) of bit width for early transition shifts, and 26/64 (approximately41%) of bit width for late transition shifts. Thus, FIGS. 15-17 showthat, for example, for systems with 11 nm bit width, all of thetransition shifts coming from adjacent bits can be compensated bymodulating the peak temperature within a ±50° K. temperature range inHAMR systems.

The temperature profile can be further modulated within the duration ofthe bit length by modulating the laser power faster than the bitfrequency of the system. In this way, the system response shape andpossible unwanted excessive ISI in the system can be controlled bydeliberately induced controlled temperature changes.

Laser modulation can be used to compensate data dependent nonlinearshifts in HAMR systems. Unlike conventional write precompensationmethods for magnetic recording systems (where transition shifts arecorrected by writing the transition slightly earlier or slightly later),this invention modulates the temperature transferred into the system bymodulating the laser power. The method can be used alone to achieve thewrite precompensation functionality, or can be combined with theconventional methods in today's read channel chips.

In another aspect, this invention adjusts the transition locations bymodulating the laser power (i.e., the temperature profile in thesystem), thus precompensating the NLTS effect of adjacent bits duringthe write process. Simulation results, obtained using two different HAMRsignal modeling simulation environments (Thermal Williams-ComstockModel), and a micromagnetic model, show that the invention cancompensate all the nonlinear shifts specified in current read channelchip architectures by just modulating the laser power within ±50° K.temperature range.

In order to reduce possible unwanted effects created by temperaturechanges and/or to make the system more robust, the invention can use asmall temperature range that can be implemented on read channel chips.

The laser power can be further modulated within the duration of bitlength. This can shape the system response and make the system lesssensitive to possible unwanted effects. For example, by modulating thelaser power three times faster than the channel baud rate of thechannel, the excessive ISI in the system can be reduced.

Other disturbances besides HMS could reduce the recording quality.Examples are media and head efficiencies, ambient temperature, componentaging, etc. To compensate these disturbances, another embodiment of theinvention compares the read signal with an optimum signal parameter. Themethod comprises: comparing a read signal with an optimum read signalparameter to produce a control signal, and modulating a source ofelectromagnetic radiation to heat a portion of a storage medium inresponse to the control signal. A look-up table can be used to alter thelaser modulation. For example, the look-up table can map the controlsignal to a power of the source of electromagnetic radiation.

While the invention has been described in terms of several examples, itwill be apparent to those skilled in the art that various changes can bemade to the described examples without departing from the scope of theinvention as set forth in the following claims.

1. An apparatus comprising: a storage medium; a source ofelectromagnetic radiation; a transducer configured to direct theelectromagnetic radiation to the storage medium; and a control circuitfor modulating a power of the source of electromagnetic radiation toheat a portion of the storage medium in response to a static deviationof a spacing between the transducer and the storage medium, wherein thecontrol circuit averages a signal produced in response to a plurality ofsignal pulses representative of magnetic bits on the storage medium andcompares the average to a predetermined value to determine the staticdeviation of a spacing between the recording head and the storagemedium.
 2. The apparatus of claim 1, further comprising: an adaptivegain controller; and a processor for computing the average from anoutput of the adaptive gain controller.
 3. The apparatus of claim 1,wherein the average is determined over a plurality of sectors.
 4. Theapparatus of claim 1, wherein the plurality of signal pulses arerepresentative of magnetic bits on the storage medium at a constantradius on the storage medium.
 5. The apparatus of claim 1, furthercomprising: a look-up table for mapping the static deviation of aspacing between a recording head and the storage medium to the power ofthe source of electromagnetic radiation.
 6. A method of compensating astatic deviation of a spacing between a transducer and a storage medium,the method comprising: producing a control signal representative of thestatic deviation in spacing between the transducer and the storagemedium; and modulating a power of a source of electromagnetic radiationto heat a portion of the storage medium in response to the controlsignal, wherein the control signal is produced in response to aplurality of signal pulses representative of magnetic bits on thestorage medium.
 7. The method of claim 6, wherein the control signal isproduced by comparing an average of the plurality of signal pulses to apredetermined value to determine a static deviation of a spacing betweenthe recording head and the storage medium.
 8. The method of claim 7,wherein the average is determined over a plurality of sectors.
 9. Themethod of claim 6, wherein the plurality of signal pulses arerepresentative of magnetic bits on the storage medium at a constantradius on the storage medium.
 10. The method of claim 6, wherein thepower of the source of electromagnetic radiation is selected from alook-up table.